The invention relates to an apparatus for controlling an engine/transmission unit of a motor vehicle. The apparatus includes an operating member for setting a power setpoint and a data storage which contains characteristic power curves of the engine.
In order to obtain predetermined operational behavior of engine--transmission assemblies also known as the drive train or power train of a vehicle, especially the combination of diesel engines and stepless, hydrostatic transmissions used in heavy-duty trucks, it is known to perform a simultaneous control of the operation of the engine and the stepless transmission.
The British Pat. No. 1,258,591 describes a control unit for an adjustable hydrostatic transmission powered by an internal combustion engine which employs as its control element a three-dimensional cam having three spatially curved control surfaces, each of which is followed by a sensor. The sensors engage suitable actuators which control the fuel supply of the internal combustion engine and the displacement of a hydrostatic pump and motor. The cam is movable axially and in rotation, one of these motions being caused by a control lever while the other is performed by an actuator controlled by the high pressure in the hydrostatic transmission. The control process is thus a mechanical one, rather than an electronic one, and it is difficult and expensive to produce the control surfaces of the three-dimensional cam with an exactness which meets the demands for control precision. The continuous mechanical contact of the sensors with the control surfaces also results in rapid wear of the cam.
U.S. Pat. No. 3,324,740 describes a further control apparatus in which the driver's actuation of the gas pedal sets the transmission ratio of the transmission as well as the speed (rpm) of the engine. A first control unit maintains the desired engine speed by positioning the throttle valve while a second control unit adjusts the transmission ratio. A further control unit is provided to prevent stalling of the engine by adjustment of the transmission ratio when the drive shaft is overloaded for a given transmission ratio. The latter control unit also adjusts the transmission ratio when the engine speed becomes excessively high. However, the further control unit becomes effective only if the desired engine speed cannot be maintained by the associated actuator, whereas, in normal operation, the transmission ratio and the engine speed is selected by the driver, the speed being set in conventional fashion by means of the carburetor setting.
The German Patent Disclosure Document DE OS 2 340 841 describes an apparatus for the common control of a drive motor and a stepless hydrostatic transmission on the basis of the command value for the output torque and of the instantaneous engine speed derived from the aspiration rate and the operational pressure. A tacho-generator monitors the engine speed and its signal serves for shifting the null point during the application of the instantaneous speed signal to a control unit which generates the control signal for the inclined planes of the hydrostatic transmission.
It is a common disadvantage of the aforementioned apparatuses that they do not, or only approximately, make use of the operational engine data for obtaining minimum fuel consumption and thus cannot guarantee operation of the vehicle with optimized consumption and correspondingly low toxic emission.
The German Patent Disclosure Document DE OS 2 712 327 further describes a method for the automatic control of motor vehicles in which the throttle valve in the carburetor of the drive motor is controlled in dependence on the position of the gas pedal, via a first characteristic data memory. A second characteristic data memory is used to derive a nominal engine speed value from the gas pedal position; this value is compared with the actual engine speed value and the difference is applied to a servo-mechanism for adjusting the stepless transmission. One disadvantage of this method is the fact that the control of the throttle valve is superimposed on the transmission control, i.e., the correcting variable for the transmission follows the changes in engine speed due to the changes in throttle valve position. Due to the fact that the throttle valve control operates substantially faster than the transmission control, there occurs a hysteresis effect between the two control processes, preventing the operation of the drive motor along the characteristic curve for minimum fuel consumption.
Finally, the German published patent application DE AS 2 328 112 describes a controller for regulating the fuel consumption and the transmission ratio for the drive means of a motor vehicle in which the gas pedal sets a nominal engine speed value which is compared with the actual engine speed value and the difference is used to control magnetic valves which change the effective transmission ratio of the stepless transmission. At the same time, the difference of the engine speed values is used to control a magnetic valve which sets the position of the throttle valve along a given characteristic curve. This apparatus shares the disadvantage of only incompletely making use of the optimum operational vehicle conditions necessary for minimum fuel consumption. Furthermore, it is a common disadvantage of all of the known systems to make possible at best operation at approximately favorable fuel consumption but not to provide the possibility for obtaining the full output power of the motor vehicle when required. Inasmuch as the controlled operation of a motor-transmission unit under conditions of optimized fuel consumption necessarily entails the possibility of a speed reduction for certain load changes, dangerous situations may arise, for example during passing, when the driver depends on the availability of high power from the engine.